feat: add CP857 (Turkish DOS) to supported document encodings

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Summary

This PR adds CP857 (Turkish DOS) to VS Code's supported document encodings, resolving the long-standing request in #300041 (160+ upvotes).

Problem

CP857 (Code Page 857) was the dominant encoding used in Turkey until the 2000s. Many legacy Turkish projects, government databases, and archived files still use this encoding. Currently, VS Code users working with CP857-encoded files cannot select this encoding from the encoding picker, forcing them to use workarounds or external tools.

While CP857 is already recognized in VS Code's terminal encoding detection (src/vs/base/node/terminalEncoding.ts), it was missing from the document encoding picker (SUPPORTED_ENCODINGS).

Changes

src/vs/workbench/services/textfile/common/encoding.ts:

• Added cp857 entry to the SUPPORTED_ENCODINGS map
• Grouped it with the existing Turkish encodings (Windows 1254 at order 32, ISO 8859-9 at order 33)
• Assigned order 34, shifting subsequent encodings by +1
• Labels follow existing conventions: 'Turkish (CP 857)' (long) / 'CP 857' (short)

Why this works without further changes

• @vscode/iconv-lite-umd already supports CP857 natively — no dependency updates needed
• The encoding picker UI automatically picks up any entry in SUPPORTED_ENCODINGS
• The toNodeEncoding() and encodingExists() functions handle CP857 correctly via iconv-lite's normalization

Testing

• Verified that @vscode/iconv-lite-umd recognizes 'cp857' as a valid encoding
• The change follows the exact same pattern as all other DOS code page entries (CP 437, CP 850, CP 852, CP 865, CP 866)
• No breaking changes — this is purely additive

Before / After

Before	After
Turkish section shows only Windows 1254 and ISO 8859-9	Turkish section now also shows CP 857
CP857 files cannot be opened with correct encoding	CP857 files can be properly decoded and re-encoded

Fixes #300041

[vs-code-engineering]

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The following users are being notified based on files changed in this PR:

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• src/vs/workbench/services/textfile/common/encoding.ts

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